1. Field of the Invention
The present invention relates to a radio communication system, more specifically to a radio communication system of a multistate quadrature amplitude modulation (QAM) type.
2. Description of the Prior Art
In a radio communication system, data is sent from a transmission system over a digitally modulated carrier wave to a receiving system, where the carrier wave is demodulated and the data is regenerated. Various systems are used for the digital modulation.
One of these is the multistate QAM system. In a QAM type radio communication system, both a phase component and an amplitude component of the carrier wave are modulated. Many modulation points corresponding to the data are arranged on one plane. Therefore, a great amount of data can be transmitted and transmission capacity can be considerably increased.
The number of modulation points formed (states) in the QAM system can be selected from such numbers as 4, 16, 32, 64, 128, etc. In the case where a high number of modulation points are used, it is important that a standard carrier wave be regenerated for synchronous detection during demodulation of the original data.
The most often used multistate QAM system is the 16-state QAM system. The regeneration circuit for the standard carrier wave in this case is conventionally either a selective control type, which extracts a necessary component with the phase of the carrier wave from the modulated signal, or a reverse modulation circuit, which produces a regenerated carrier wave by further modulating the modulated data and multiplying it with the received wave. However, in the former, when the error rate is high, a carrier wave having a good signal-to-noise (S/N) ratio cannot be obtained and, in the latter, the circuit construction becomes complex. In both cases, the greater the number of modulation points (states), the worse the S/N ratio. Further, a ziehen phase of the carrier wave becomes indefinite, so that a differential coding and decoding logic process must be used.
There has been proposed in Japanese Patent Application Nos. 57-231635, 57-231636, and 57-231634, filed on Dec. 29, 1982, and corresponding U.S. application Ser. No. 566,401; a method by which frequency components near the carrier wave are removed by filters from the spectrum of the transmission side and the pilot signal is further superposed onto the carrier wave in the transmission side, and regenerates the carrier wave in the receiving side. In this method, however, regeneration of the removed spectrum in the receiving side is difficult.
In a multistate QAM system which previously suppresses the low-frequency component of the transmission spectrum when generating the carrier wave, the carrier wave is frequency-modulated by a service channel signal, that is, a composite modulation system is used. In this system, however, a carrier wave having a good quality cannot be obtained and the error rate of the data becomes higher.